A light emitting diode made of a nitride semiconductor such as GaN can emit ultraviolet light or blue light, and can also be configured to emit white light by using a fluorescent substance. Among such white LEDs, an LED that can emit white light with high output power is applicable for illumination purposes.
Sapphire is generally used to form a growth substrate for a nitride semiconductor. Meanwhile, a sapphire substrate has insulating properties, so that an n-electrode and a p-electrode should be formed on the same surface (growth layer side) of the substrate. Thus, an electrode pad and an interconnect line on the n side, and those on the p side should be formed in different regions. The electrode pads on the n and p sides may hinder the optical output and increase light absorbance. In general, when the electrode pads are provided on the same surface, series resistance may increase more than the case where the electrode pads are provided on opposite surfaces. This can increase the driving voltage applied to the LED. Additionally, the heat conductivity of the sapphire substrate is low, so that the sapphire substrate has poor heat dissipating properties. Thus, a sapphire substrate cannot be used suitably in a device to be supplied with a large current.
In view of these circumstances, an improved device structure has been developed in recent years. In this device structure, a sapphire growth substrate is removed by laser lift-off (LLO) or polishing to expose a nitride semiconductor layer (being an n type semiconductor layer), and an n electrode is formed on the exposed nitride semiconductor layer, thereby arranging the n electrode and a p electrode at respective opposite positions (see Japanese Patent Application Laid-Open No. 2010-056458, for example).
FIG. 1 is a schematic sectional view of the structure of a conventional nitride semiconductor light emitting element (LED element) 201.
In a process of manufacturing the LED element 201, a GaN device structure layer 202 composed of an n-type GaN layer, an active layer, and a p-type GaN layer is first formed on a transparent growth substrate made of sapphire and the like. Then, a reflective electrode 203 made of Ag and the like, and a first adhesive layer 205 are formed sequentially. The first adhesive layer 205 has a side surface vertical or forward tapered with respect to the growth substrate. Meanwhile, an Si support substrate 210 is prepared separately while an insulating layer 209 and a second adhesive layer 206 made of AuSn and the like are formed on the surface of the support substrate 210.
Next, the growth substrate is turned upside down and the first and second adhesive layers 205 and 206 are bonded to each other. This forms a fusion layer 226 so that the two substrates are bonded. Then, the growth substrate is removed by LLO. Next, an insulating film 207 is formed except for the upper surface of the device structure layer 202 exposed as a result of removal of the growth substrate and part of the fusion layer 226 to become a p electrode 212.
Then, an n electrode (extraction electrode) 211 and an interconnection electrode 208 are formed. The n electrode 211 is disposed to extend along one side of the outer circumference of the device structure layer 202 while being distanced a certain space from the device structure layer 202. The interconnection electrode 208 is disposed to extend from the upper surface (light emission surface) of the device structure layer 202 to cover a side surface of the same, thereby connecting the n type GaN layer and the n electrode 211.
Regarding the conventional LED element 201, the fusion layer 226 is formed by forming the first adhesive layer 205 so as to have a side surface vertical or forward tapered with respect to the growth substrate, turning the first adhesive layer 205 upside down, and bonding the first and second adhesive layers 205 and 206 to each other. As a result, an end face structure of the fusion layer 226 becomes vertical or reverse tapered with respect to the support substrate 210 as shown in FIG. 1 (meaning that an angle formed between the surface of the fusion layer 226 on the side of the light emission surface and the side surface of the fusion layer 226 becomes 90 degrees or smaller). Thus, the interconnection electrode 208 disposed on an edge of the fusion layer 226 (part surrounded by dashed line in FIG. 1) between the surface on the side of the light emission surface and the side surface of the fusion layer 226 may be disconnected during the manufacturing process or after the manufacture, leading to reduction of the reliability of the LED element 201.